The present invention relates to a rotation angle detection apparatus for detecting the steering angle of a steering wheel in, for example, a vehicle.
In automobiles having electronic control suspension systems and vehicle traveling control systems, known as vehicle stability control (VSC) systems and electronic stability program (ESP) systems, a rotation angle detection apparatus is incorporated in the steering column so as to detect the steering angle of the steering wheel. Known types of rotation angle detection apparatuses include those that detect the absolute angle and others that detect a relative angle.
Japanese Laid-Open Patent Publication No. 2002-98522 describes a conventional rotation angle detection apparatus that detects the absolute angle. The rotation angle detection apparatus is provided with a rotation plate, three photosensors for detecting angles during one rotation of the rotation plate, and a single magnetoresistance element for detecting the number of rotations of the rotation plate. Specifically, three slits having mutually different patterns are provided at three locations in the circumferential direction of the rotation plate. Three photosensors are arranged at positions corresponding to the three slits. Each photosensor outputs a detection signal in accordance with whether or not a slit is at the associated position. Three-bit codes of the detection signals output from the three photosensors are determined so as to not be repeated during a single rotation of the rotation plate. The rotation angle (absolute value) of the rotation plate is determined based on the three-bit codes.
In this type of rotation angle detection apparatus, the rotation angle of the rotation plate is detected as an absolute value. Thus, even if the detection apparatus stops functioning when the rotation plate is rotated, the absolute value of the rotation plate is detectable when the apparatus starts functioning again. However, a rotation angle detection apparatus that detects the absolute angle has a shortcoming in that many devices are required for angle detection and in that it is difficult to design the pattern of the slits on the rotation plate.
Japanese Laid-Open Patent Publication No. 2000-46536 describes a conventional rotation angle detection apparatus that detects the relative angle. The rotation angle detection apparatus is provided with a rotation plate having a plurality of equally spaced slits and three photosensors for detecting the rotation angle of the rotation plate. Each photosensor outputs a reflected binary code in accordance with whether or not a slit is at an associated position. The rotation angle detection apparatus relatively calculates the rotation angle of the rotation plate by adding or subtracting the number of changes of the reflected binary code output from each photosensor from the angle data that is used as a criterion.
In the rotation angle detection apparatus that detects the relative angle detecting type, the rotation angle of the rotation plate may be detected by using two photosensors, and simple slits are provided on the rotation plate. This facilitates the designing of the slit pattern.
In a rotation angle detection apparatus that detects the relative angle, if the detection apparatus stops functioning when the rotation plate is rotated, the rotation angle of the rotation plate immediately prior to the stopping may differ from the actual rotation angle of the rotation plate when the detection apparatus starts to function again. Therefore, the rotation angle detection apparatus must continuously detect the rotation angle of the rotation plate even when the automobile ignition is turned OFF. However, when the rotation angle detection apparatus is operated continuously in a state in which the vehicle ignition is turned OFF, there is concern with drain of the battery due to large dark current. Thus, when the ignition is turned OFF, the rotation angle detection apparatus is intermittently operated. This reduces the power consumption of the rotation angle detection apparatus and prevents the rotation angle of the rotation plate from being erroneously detected.
Specifically, in the rotation angle detection apparatus of Japanese Laid-Open Patent Application No. 2000-46536, the resolution is 1.5 degrees, and the output phase differential of the photosensor is 1.5 degrees. Therefore, the cycle during which the rotation angle detection apparatus is intermittently operated (sampling cycle) must be set at a time interval which is at least shorter than the time required for the rotation plate to rotate 1.5 degrees. For example, when the maximum tolerable rotation speed of the rotation plate is two rotations per second (720 degrees per second), the required sampling cycle is approximately one millisecond or less.
In recent years, however, the requirement to reduce dark current has become more severe in conjunction with the move to vehicle electronic controls and the increase in built-in battery powered devices, as well as the requirement for reducing the power consumption of the rotation angle detection apparatus when the ignition is turned OFF.